Fluoropolymers have been used in many different high performance coating applications for over forty five years due to their excellent weathering properties which include gloss retention, color retention, chemical resistance, water resistance, chalking resistance and resistance to biological growth. Poly(vinylidene fluoride) (PVDF) has performed very well in architectural applications, where both excellent appearance and substrate protection must be maintained over long periods of time. The majority of fluoropolymer-based coatings are solvent based, producing high levels of volatile organic compounds (VOCs). Water-based coatings based on a fluoropolymer latex are attractive because they offer an environmentally friendly (low VOC) route to high performance coatings. For example, environmentally friendly coatings based on KYNAR AQUATEC® polyvinylidene fluoride-acrylic hybrid latexes (from Arkema Inc.), having a solid content of 42-45 wt %, exhibit better weatherability, dirt pick up resistance, stain resistance and chemical resistance than conventional acrylic, urethane and silicone coatings.
Because of their excellent properties, it is desirable to use fluoropolymer-based coating compositions to protect a variety of substrates. However, for some substrates such as glass the adhesion of the fluoropolymer based coating composition is quite limited due to the super-hydrophobicity of fluoropolymers. To achieve satisfactory adhesion of a fluoropolymer coating to glass, it is generally necessary to utilize some type of adhesive materials or to carry out costly and time-consuming glass treatments, such as surface etching, etc.
There is a need for an aqueous fluoropolymer-based coating composition that exhibits all of the desirable properties associated with a fluoropolymer coating and also has excellent adhesion directly to glass.
U.S. Pat. No. 4,879,345 describes a solvent-based coating composition based on polyvinylidene fluoride (PVDF) resin, that offers improved adhesion to glass substrates. To achieve good adhesion, the coating requires functional organosilanes and an adjuvant resin that reacts with the functional group on the organosilanes. The composition exhibited a good dry adhesion (adhesion tested before treatment with high humidity or water) directly to glass, but the wet adhesion (adhesion tested after treatment with high humidity or water) was unacceptable. The wet adhesion of the working examples after only 30 minutes soaking in boiling water was already less than 100%.
CA2091847 discloses a fluoropolymer based coating composition for glass substrates, which consists of a fluoropolymer resin, an organosilane and/or an organo-titanate, an inorganic pigment and a thermoplastic polyacrylate resin that has no functional groups reactive with the organosilane or organo-titanate. This composition is also organic solvent-based. The glass must be primed first with a clear coat which is a mixture of an organosilane and an organo-titanate. In addition, it is believed that the glass-to-coating bonding is a result of direct chemical bonding between the glass and some of the inorganic pigment particles. Therefore, CA2091847 applies only to coatings containing inorganic pigments and it does not apply to a clear top coat. Furthermore, wet adhesion of the coatings in CA2091847 was not reported. Both prior arts are solvent based coatings.
U.S. Pat. No. 5,852,106 describes an aqueous polyurethane binder for coating compositions and to their use for coating glass, preferably glass bottles. The binder contains a water-dispersed hydroxy-functional polyurethane dispersion and water dispersed blocked aliphatic polyisocyanate resin, wherein the equivalent ratio of blocked isocyanate groups to hydroxyl groups is at least 0.8:1. EP 519074 describes an aqueous glass coating composition that is applied in two coats, wherein the topcoat substantially contains three main components: a polyurethane dispersion, an aqueous epoxy resin and an aqueous melamine/a formaldehyde resin. While these, fully cross-linked coatings demonstrate excellent adhesion to glass, they do not have excellent weathering properties, chalking resistance and chemical resistance associated with fluoropolymer compositions.
Through diligent investigation it has been found that an environmentally friendly (low VOC) aqueous hydroxyl-functional fluoropolymer coating composition can be adhered to glass that has been primed with a polyurethane primer, and partially and not fully cross-linked. The multi-layer coating composition provides excellent weathering properties, chalking resistance, chemical resistance and dirt pickup resistance. The coating composition is especially useful directly on glass substrates to provide a protective barrier with superior dry and wet adhesion.